A Storage Virtual Appliance (SVA) is a special purpose virtual machine to manage shared storage in virtualized systems. A data transfer between a virtual machine and a SVA is performed through standard protocols as for example Network File System (NFS), Common Internet File System (CIFS), Internet Small Computer System Interface (iSCSI), etc.
There is a significant performance overhead, in terms of additional CPU cycles and latency, when compared to the traditional approach in which the storage virtualization service is provided directly by the hypervisor. Data copy to and from network buffers consumes a lot of CPU cycles when the hypervisor redirects the storage I/O (input-output) from a VM to a SVA using iSCSI or a similar TCP/IP based protocol. When writing data to storage, the VM sends the data to the hypervisor, which then copies it from the VM's memory onto the hypervisor network buffers and delivers the data over the virtual network to the SVA. The SVA network driver copies the data from the hypervisor buffers into the SVA's private buffers. The SVA again sends the same data to the hypervisor to be written to the physical disk. This involves one more level of copy. This makes sense if the SVA and VM were in two different physical hosts. However, if the SVA and the VM are co-resident, this adds unnecessary copies and delays in terms of context-switching (between the VM-hypervisor, hypervisor-SVA, SVA-hypervisor).